Technical Field
The invention concerns a method for determining the current filling of a lung in accordance with claim 1, and a method for assisting in radiotherapy during movement of the radiation target due to breathing in accordance with claim 8.
Irradiating tumours in the area of the lung, liver and kidneys is made significantly more difficult by the fact that these areas experience movement due to breathing. Only insufficient systems currently exist for detecting and/or measuring this movement. In present clinical practice, patients are “mapped” in CT or in a transillumination image, i.e. a planning data set is produced, and a target volume marked in and localised within a patient co-ordinate system (for example, using markings on the skin). The problem with this, however, is that the position of said target volumes is affected by the amount of air which the patient has in his lungs while the CT is being taken. The position of the target volume established during planning is therefore only valid for precisely one lung filling. FIG. 1 illustrates the movement of the target volume according to variations in lung filling when the patient is breathing freely.
The patient's lung filling is the quantity which affects the position of the target volume within the patient's body. Tracking the patient's lung filling over time establishes that it changes with each breath (this is the approximately 0.5 liters of air which the patient breathes in and out). Tracking the lung filling over a longer period of time, however, establishes that the breath maximum, minimum and median likewise change over time, as illustrated in FIG. 2, which shows lung filling variation over time.
It cannot therefore be predicted for any selected point in time (or short period of time) in the future, where the maximum, minimum and median of a patient's lung filling will be situated. The problem exists that a 3D planning data set cannot be transferred to the state of the patient at the time of radiation exposure. If the planning data set has been taken with the patient holding his breath (this normally being the planning CT), this data set represents the patient and the target volume at a particular lung filling. In order to be able to transfer these data to the state of the patient at the time of radiation exposure, the lung fillings at the two points in time would have to correspond precisely. If the patient is able to breathe freely during radiation exposure, this is the case at most twice in each breath. Since patients—who have to hold their breath for about 10 to 20 seconds for the planning CT—have a tendency just before it to breathe in or out more strongly or more weakly than in normal breathing, it can also occur that during radiation exposure the patient at no point has the same lung filling as in the reference (planning) data set. In this case, the position of the target volume is at no point known, and pin-point irradiating is thus impossible.
In many medical practices, it would therefore be advantageous to be able to detect the patient's breathing precisely, in order for example to be able to track the position of organs or tumours affected by breathing. For this purpose, methods have already been developed for detecting changes in external parameters of the patient, so allowing the patient's breathing to be detected. The patient's breathing can also be tracked by spirometry. The problem with these solutions, however, is that breathing can only be tracked as long as the patient remains in contact with the system. If, however, it is necessary to compare the patient's breathing at two, somewhat far apart, points in time, the above-named systems would have to be permanently in contact with the patient. Since in many medical practices this is impossible or undesirable, such uninterrupted contact is not, as a rule, available. In all hitherto known methods, the problem arises that it is not possible to find a zero point or another absolute reference point again with satisfactory precision. Where measuring has been interrupted between the acquisition of readings, these readings cannot then be directly compared. The unsatisfactory precision is caused for instance by sebaceous layers, and the reference markings arranged on them, slipping, or in spirometry by the inability to reproduce the maximum and minimum lung filling.